processes of ephemeral gully erosion - PubAg - USDA

PROCESSES OF EPHEMERAL GULLY EROSION Ja'.:er Casali 1 , Sean J. Bennett, and Kerrv M. Robinson'

ABSTRACT The formation of ephemeral gullies can si gnificantly increase soil loss from agricultural lands and severely impac: farm procuctivity. Erosion prediction technology and conservation management techniques would be izreatl'. Improved if the contribution from e phemeral gullies could be more accurately quantified. Field research n Nlississippi. U.S.A. and Spain has revealed three categories of ephemeral gullies. Classic ephemeral zullies tornied by concentrated t10 erosion from runoff occurring within the same field. Drainage epheme ral gullies formed by concentrated flow erosion from runoff originating from areas upstream of where :re gully occurs. Discontinuity ephemeral gullies formed in areas where mana gement prac:ices ha e created a sudden chan ge in slope, such as field boundaries adjacent to roads. Despite the larac differences ri climate, watershed size, hvdroloay, and geography, the ephemeral gullies observed in Spain were mor—phologically similar to those in Mississippi. Using an experimental flume, ephemeral gulls erosion proceeded primarily through bed incision, gully widening, and bank steepening, and total sediment load de pended upon whether the flow was detachment- or transport-limited. Ke y Words:

Ephemeral gully: Soil loss; Erosion prediction technology; Conservation management; Bed incisions; Total sediment load

I INTRODUCTION Ephemeral gullies are small erosional channels on agricultural landscapes caused by the concentration of overland flow typically between two opposing slopes (a hollow), often formed during a single rainfall event. Since the scoured soil volume is not very large within these gullies, farmers can easily refill them. In general, ephemeral gullies can reappear at or near the same location on a yearly basis because the surface topography of the field does not change appreciably. Most ephemeral gullies occur on cultivated fields with highly erodible soils, with little or no crop residue cover, and where crop harvest disturbs the soil (USDA-NRCS, 1997). Ephemeral gully erosion is not accounted for in current soil-loss assessment programs, but its contribution and importance to total soil losses has long been recognized. The USDA-NRCS estimated the ratio of ephemeral gully erosion to nh and sheet erosion, and values range from 21% for New York and 274% for Washin gton, v.ith an average of 78% for the 19 states surveyed (Table I). In actively eroding areas, ephemeral gullies typically contribute about 30% to the total soil loss, but can reach as high as 100% (Laflen et al., 1985 b; Spomer and Hjelmfelt, 1986; Thomas et at. 1986; Thorne et al., 1986; Grissinger and Murphey, 1989: Lentz et al. 1993; Table 2). Ephemeral gully erosion also impacts the Loess Belt and Mediterranean region of Western Europe (Poesen and Govers, 1990; Table 2). In the U.S.A. and Europe, ephemeral gully erosion is responsible for at least 10% of total soil losses (Poesen et al., 1996). Ephemeral and permanent gully erosion is also a severe problem in Spain (Sala et al., 1991; Sala and Rubio, 1994; Casali e al., 1998), and parts of Spain are at risk of desertification (Poesen, 1995; Rubio, 1995). Accurate assessment of ephemeral gully erosion is limited by a number of factors. At present, few field studies have provided viable data on gully erosion rates, and these studies tend to be restricted in both time and space. Accurate rainfall data are also needed to constrain erosion rates and projected soil losses. Ephemeral gullies can form by a variety of causes. Smith (1993) identified the following critical Parameters for gull y development: (1) a critical slope length and slope gradient that is dependent upon Slope characteristics and crop row direction, (2) occurrence and depth of a fragipan, (3) agricultural Dep artment of Projects and Rural Engineering, Public Universit y of Navarra, 31006 Pamplona, Navarra, Spain National Sedimentation Laboratcrv. USDA-ARS, P.O. Box 1157, Oxford, MS 38655, USA Plant Science and Water Conservation Research Laboratory, USDA-ARS, 1301 N. Western St., Stillwater, OK 74075, USA Note: The manucript of this paper was received in march 1999, Discussion open until March 2001. International Journal of Sediment Research, Vol. 15, No. 1, 2000 9 pp. 31-41 -31 -

practices, principall y row direction and timin g of cultivation, and (4) timing and total amount of precipitation. Linear features of the landscape like plot borders, lanes, tractor rows, or furrows can also promote ephemeral gully development (Laflen. 1985; Poesen and Govers. 1990; Casali, 1997; Casali et al.. 1998). Moreover, headcut development appears to be an important triggering mechanism for gully erosion, particularly in classic and discontinuity gullies (see below: Smith, 1993: Casali et al.. 1998). Assessment ofe p hemeral g ull erosion rates in selected areas of the U.S.A. (from USD.A-NRCS. 997) Estimated Annual Measured Ephemeral Ephemeral Gully Erosion Location Sheet and Rill Erosion Gulk Erosion as a Percentage (%) of (k ,-,!m -V) (ko!ni-v) Sheet and Rill Erosion Alabama 0.573 0.342 60 Delaware 0.038 0.093 245 Illinois 0.261 0. (91 73 Iowa 0.353 0.110 3' Kansas 0.807 0.294 36 Louisiana 0.65. 0.222 34 Maine 0.41 0.189 46 Maryland 0.195 0.147 75 Michigan 0.172 0.045 26 Mississippi 0.646 0.275 43 New Jersey 0.246 0.191 78 New York 0.873 0.185 21 North Dakota 0.277 0.130 47 Pennsylvania 0.093 0.065 70 Rhode Island 0.331 0.136 41 Vermont 0.165 0.224 136 Virginia 0.477 0.470 98 Washington 0.025 0.069 274 Wisconsin 0.289 0.154 53 Table I

There is no unique solution to prevent or mitigate ephemeral gully formation and erosion. Reduced tillage is an excellent soil conservation practice that can reduce gully erosion (Laflen et al., 1985a; Spomer and Hjelmfelt, 1986; Dc Ploe y , 1988), but more specific measures such as retention structures, diversions, waterways, terraces, and underground outlets are often required (Foster, 1986; USDA-NRCS, 1997). Vegetation management programs appear to be well suited for controlling ephemeral gully erosion. Smith (1993) noted that residue often helped reduce erosion by clogging rows and gullies and actually inducing deposition. The combination of reduced tillage and vegetation barriers such as stiff grass hedges constructed within gullies have decreased the headward advance of gullies in cultivated fields of northern Mississippi (Dabne y et al., 1997). Contoured grass buffer strips and strip-cropping offer additional alternatives to mitigate ephemeral gully erosion (USDA-NRCS, 1997). Table 2. Summary of relevant data concerning ephemeral gully erosion. Methods used to measure gullies include: simple volumetric measurements with profilers and tapes, conventional photography, aerial photogrametrv, and digital terrain models. All studies in U.S.A. except (2) and (6) use Universal Soil Loss Equation to estimate rill and interrill erosion. Data ranges are given in parentheses. Sources of data, soil types, and land management: (1) Miller (1982), soil hydrologic Group A and B; (2) Spomer and Hjelmfelt (1986), loess, conventional till: (3) Laflen (1985), loess and glacial till; (4) Thomas et al. (1986), Thomas and Welch (1988), sandy loam, soybeans, conventional till; (5) Gnissin g er and Murphey (1989). loess, soybean, conventional till; (6) Lentz et al. (1993). loess or g lacial-till- loess, corn and soybeans, conservation till; (7) Smith (1993), boessial silt barns with fragipan. soybeans or corn, conventional till; (8) Moore et al. (1988), bare, salodic loam; (9) Auzet et al. (1993) variable crops and managements; (10) Vandacle (1993), loess, silty loam, variable crops; (11) Vandaele and Poesen (1995) loess. silty loam, variable crops; (12) Poesen et al. (1996), sandy loam, 20-50% rock fragments, inactive; (13) Vandacle et al. (1996) a - loess, silty loam, variable crops; b - lithosol. >30% rocks, winter wheat and barley; (14) Casali et al. (1998) loam or silt-loam, winter grains, conventional till; (15) Hidal go et al (1998), clayey soil. -32-

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Table 2 Summary of relavant data concerning ephemeral gully erosion Slope (%) Drainatze Soil loss. Total Soil Loss. % of total Watershed Ephemeral rill/sheet soil loss Source Location (W) or gully Area (ha) Gullies Soil loss erosion due to ephemeral (G) (W: G) (ka/m1-v) (k,-,/m -Y) (k/m gullies

United States

(I) Alabama na. na. 0.80, 1.45 1.34, 2.90 0.54, 1.45 60, 50 Iowa 4.0-14.0 (W) 24.3 W) 1.70, 068 8.9, 0.62 7.20, 0 19, 100 (2) Iowa 2.0-11.0 (W) I'' (W) 0.19-0.73 0.97-3.75 078-3.02 9-20 (3) Georaia 4.5 (G) 5.3 (W) 4.00. 5.06 10.73 6.20 42 (4) 2.0 (G) rvltssss;ppi na. 2.45 098 (5) l.9(\) 1.47 60 (6) Minnesota 3.4-6.1 (W) na. na. 7.4 (W) 0.30 na. (0.15-0.54) (7) Mississippi 0.8-2.0 (G) 6.04 4.36 2.2 (G) 1.68 36 (1.2 1-2.02) (4.51-10.3) (0.90(16-67) 8.74) .-1 ustrulia Australia 12.5 (W) (8) 7.5 (W) 1.30 na. n.a. n.a. (9)

Europe

France 1.9-7.9 (W) Ca. 650 (W) 0.20 0.29 0.09 72 cu. 410 (G) (0.09-0.70) (0.05-0.93) (0-0.25) (36-100) (10) Belgium gentle 170.0 (W) 0.21-0.35 0.56-0.82 0.35-0.50 37-39 (II) Belgium gentle 25.0 (W) Ca. 0.40 0.85 0.41 52 (12) Spain 3.0-25.0(W) 10.0(W) 1.26 1.52 0.26 80 (I 3a) Belgium gentle 4,000.0(W) 0.15-1.32 0.52-1.90 0.36-0.58 30-69 (I 3b) Portugal gentle 550.0 (W) 0.10-0.68 0.12-0.80 0.02-0.13 83-84 (14) Spain 0.5-9.5 (G) 88.0 (W) 0.87 n.a. n.a. n.a. Ca. 5.0 (G) (0.16-2.66) (IS) Spain n.a. 4.9 (W) 6.49 Ca. 8.83t n.a. ca. 74 4.9 (G) na.: not available t Minimum value, rill and sheet erosion not considered Despite the significance of ephemeral gully erosion, little data exist on rates of soil losses, physical characteristics of gully systems, and criteria for gully formation. Below we present the main morphologic characteristics of ephemeral gullies from Southern Navarra, Spain (Casali et al., 1998) and from central Mississippi, U.S.A. (Smith. 1993), and describe recent experimental results detailing erosion processes within pre-formed swales.

2 EPHEMERAL GULLIES IN SOUTHERN NAVARRA, SPAIN Field measurements were conducted in Southern Navarra from October 1995 to September 1996 to understand further the characteristics and erosion processes of ephemeral gullies (Casali, 1997). The study site was located 50 km south of Pamplona, and comprised 88 ha of agricultural fields within a 210 ha watershed. The slope of the cultivated fields ranged from I to 14%. The climate is continental Mediterranean, with a mean temperature of 13°C and an annual rainfall about 500 mm (Elias and Ruiz. 1986). The main crop was winter cereal, wheat or barley, sown at the beginning of October. The parent material of the soil was a mixture of clay and barns from Miocene-age sandstones. Soil texture was generally loamy, although silt-loam soils were found in the low-land areas. Soil structure was platy, which reduced infiltration rate, and crust (seal) formation was common. These soils can be considered highly erodible (Donézar et al., 1990). Ephemeral gullies were examined during one growing season. During this time, detailed topographic surveys of all gullies, and rates of erosion, growth, and d evelopment were documented. Gully cross-sections were measured with a mechanical profiler C onsisting of a 1-rn wide frame that held 50 pins. Each field measurement was photographed and later digitized. A more detailed description and discussion of these results can be found in Casali et al. (1998).

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Three distinct types of ephemeral gullies were observed within this region in Spain and these are referred to as classic ephemeral gullies, draina g e ephemeral gullies, and discontinuity ephemeral gullies. Classic ephemeral gullies formed by concentrated (low erosion from runoff occurring within the same tield. These trapezoidal g ullies formed on 6% slopes, and had top widths oO.5 m, bottom idths of 0.1 m, depths ofO.2 m, len g ths of43 rn. and width-to-depth ratios (W/D) o4 (Table 3) rahk 3 Main characteristics olephemeral gullies from studies in Southern Navarra, Spain based on one year oF data Casali et al.. 1998) and Mississipni. U.S.A. based on three years of data (Smith, 1993). Mean values are shown, and minimum and maximum values are ziven in parentheses Ephemeral UI L111V characteristic Ephemeral gulk t'.pe Numbcr of sites Number of g ullies examined Near-surface soil texture Mean annual rainfall mm) Gulls' watershed area (10' m-) Total gully watershed area 10' m') Gully slope (%) Top width (m) Bottom width (m) Depth (m) Cross-sectional area (m) Width-to-depth ratio Bank angle (degrees) Gully length (m) Eroded volume per gully (m3) Total eroded volume (mi) Soil toss per unit gully length (kglm) Soil loss rate (kg/m-y) Total soil loss due to ephemeral izullies (°/) using USLE na: not available

Southern Navarra. Spain Classic

Drainaee

loamsilt loam 50)) 7.9 (2.6-17.4) 39.4 6.3 (3.0-9.5) 0.53 (0.34-0.77) 0.14 (0.08-0.20) 0.16 (0.11-0.22) 0.08 (0.05-0.11) 4.0 (2.0-5.9) 47 (25-68) 43 (18-80) 3.18 (0.83-5.50) 0.4

loamsilt loam 500 na.

Discontinuity

4

na. 3.9 (2.9-5.0) 0.73 (0.63-0.90) 0.19 (0.08-0.35) 0.19 (0.14-0.29) 0.08 (0.05-0.10) 5.1 (4.1-6.2) 53 (40-69) 107 (52-240) 7.56 (2.43-14.73) 30.2

loamsilt loam 50)) na. na. 3.7 (0.5-9.5 0,76 (0.67-0.90) 0.28 (0.18-0.36) 0.25 (0-14-0.37) 0.11 (0.04-0.18) 3.6 (2.2-6.1) 42 (34-6!) 20 (5-43) 2.46 (0.66-7.89) 12.3

117 117 171 (63-174) (72-154) (101-283) 0.87 Ca. 2.60 n.a. (0.16-1.74) na. na. na.

Mississippi, U S A. Classic 4 5 Silt loam 1.400 1.9 21.5 1.3 0.7! (0.5 1-0.81) 0.12 (0.07-0.17) 0.14 (0.06-0.25) 0.06 (0.02-0.11) 7.7 (2.5-14.5) 61 (44-74) 30 (10-57) 1.73 (0.19-3.73) 6.2 (4.1-9.1) 77 (48-102) 1.68 (1.21-2.02) 36 (16-67)

Typically a headeut or knickpoint formed, and this area of local scour probabl y enlarg ed and deepened t'ormin g a gully. Five small watersheds covering 3 . 94 ha were affected by' classic ephemeral gully ero ion during the study period. Total volume of soil eroded by classic gullies was 10.4 m s 3 . Although cross-sections were trapezoidal, maximum cross-sectional area tended to occur in the central part of the g ull. This may be due to the persistence of detachment-limited flows in the upstream reaches compared to transport-limited flows in the downstream reaches. Drainage ephemeral gullies formed by concentrated flow erosion from runoff ori g inating from areas upstream of where the gully occurs. Typically, gully incision and development began at the upper boundary of cultivated plot downstream of the runoff source region. These trapezoidal gullies generally -34- International Journal of Sediment Research, Vol. 15, No. 1, 2000, pp. 31-41

formed on 4% slopes, and had top widths of 0.7 m. bottom widths of 0.2 m, depths of 0.2 m, lengths of 107 m. and

W/D

values of 5 (Table 3). Four izullies ol' this class were monitored during the study

period. Drainage gullies were the most active gully type in the field area, eroding 30.2 m ofsoil material. These gullies were trapezoidal in shape, but typically had greater cross-sectional areas at the most upstream section. and gully area decreased downslope. Discontinuity ephemeral gullies were found in areas where management practices created a sudden chan ge in slope, such as field boundaries adjacent to roads. Ihese trapezoidal gullies generally formed on 4% slopes, and had top widths of 0.8 m. bottom widths of 0.3 m. depths of0.3 m, lengths of 20 m, and

W/D values o14 (Table 3) These gullies were g enerall y triggered by headcuts related to small piping lubes or seepae zones across the discontinuity (Gutirrez et al.. 19% Del Valle de [.ersundi and Del Val, 1990). Discontinuity g ullies accounted for 12.3 in nt soil erosion, were trapezoidal. and showed little ari:ition in cross-sectional area along their length. Oascd on field obscratiuns in this re g ion of Southern Navarra. most gullies were formed or were i g nitieantiv reactivated during one intense winter rainfall of short duration. Althou g h near normal rainfall was ohsers ed durin g the studs period (450 mm/v). only one rainfall event \•vas associated with g ull' erosion: .lanuary 22. 906 where peak rainfall rate was 54 mm h. At that time, elevated soil moisture contents facilitated runoU, and hare soil surfaces eroded most dramaticall y . Such observations support the use of a rainstorm intensity and duration index for g ully erosion modeling. This also hi g hli g hts the importance of recording hi g h-intensit y rainfall data in areas prone to gully formation. All gullies examined had

W/D

values between 3 and 6 (Table 3). In central Bel g ium. Poesen and

Govers (1990) observed that intense rains of short duration favor the formation of gullies with while frequent long rains of small intensity produce narrower gullies with

W/D < I .

W/D > 1.

In general, broader

gullies cause greater environmental degradation because losses in productive topsoil are the greatest and these soils tend to be richer in fertilizers and pesticides. Results presented here are consistent with the proposal of Poesen and Govers (1990). Classic ephemeral gullies had smaller intermediate positions, whereas drainage ephemeral gullies had minimum

W/D

W/D

values in

in upstream sections.

Maximum soil erosion occurred when W/D was a minimum, illustratin g that bed incision and flow concentration were the critical parameters for high erosion rates. In order to characterize the susceptibility of a watershed to classical ephemeral gullies, a simple topographic index based on the area-weighted mean slope of the region, AS, has been defined by Casali et al. (1998):

AS= A"

A,•S1

(I)

where the slope of every tract with uniform characteristics S, is weighted by its respective area A, and multiplied by the total area A. Thorne etal. (1986) established a similar topographic index for identifying landscapes susceptible to ephemeral gully development. Figure 1 demonstrates the application of this index in Southern Navarra where greater soil losses are associated with higher values of AS.

3 EPHEMERAL GULLIES IN MISSISSIPPI, U.S.A. Smith (1993) conducted a detailed investigation of ephemeral gullies at four sites in central Mississippi (U.S.A.) near Jackson during October 1983 to April 1985. Climate is subtropical with a mean annual rainfall of 1400 mm. During the study period, annual rainfall rate ranged between 1590 and 1640 mm. Soils were loessial silt-barns (Memphis, Loring. and Providence series), were cultivated with soybeans or corn, and winter cover was grass and wild peas. A fragipan layer was present at most gully locations at an average depth of about 0.35 m. At each site location, Smith (1993) obtained gully cross-sections. and

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final yearly measurements were made in May just before plowing. These final yearly data are the focus of the discussion herein.

6L

I A? -

L

L

-2 3 L = L

T

-

• LaAbejera 1, 1995-96 v Cobaza 1. 1995-96 U La Matea 1, 1995-96 • La Matea II, 1995-96 A La Abejera 0, 1995-96 ...-

2 •

I 1 01 200 400 600 800 1000 1200 1400

AS

Fig. 1 Relationship between total soil losses and index AS for the period October 1995-September 1996 for five different gullied watersheds in Southern Navarra (Spain). Soil losses at La Abejera 0 watershed were estimated based on measurements, daily rainfall data, and management practices (from Casali et al., 1998).

Flow Control Valve

Section A-A Compacted Cohesive Soil

Fig. 2 Large-scale outdoor flume at the USDA-ARS location in Stillwater, OK showing flow discharge system, inlet region, and pre-formed swales. -36- fl

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All gullies described by Smith (1993) are considered classic ephemeral gullies. These trapezoidal gullies generall y formed on 1% slopes, and had top widths of 0.7 m, bottom widths of 0.1 m. depths of 0.4 m, lengths of 30 m, and W/D values of 8 (Table3). Many eullies initiated one-third to one-half theay d own the slope, and then extended and grew in both directions. In general, maximum ephemeral gully erosion correlated with maximum precipitation events. Gully erosion was divided into two phases during the year (Smith, 1993). Following May plowing, rainfall in June promoted gully erosion prior to the development of crop canopy. From July to October, canopy cover and crop g rowth allowed the gullies to refill and anneal. After the November harvest, gullies were most vulnerable and could reactivate. From November to April, maximum gully erosion and extension occurred, coinciding with the hi g hest and most frequent precipitation of the year. Fifteen relatively small watersheds covering 2.2 ha were affected by ephemeral gully erosion and total volume of soil eroded avera g ed 6.2 0.24

0.24

0.00 Section 2 10.7 in 0.24 0.08

C

0.00

Th-7T I Section 3; 13.7m

0.24

:

Section 4; I6

0.24 Original After i.i8hj Section 5; 18.3 0.00 - -After 2.07 h :

I

I

0.00

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

rn

0.8 0.9 1.0

Distance from a datum (m) Fig. 3 Time variation in ephemeral gully morphology for the 6/1/98 experiment. The position of each cross-section is given as distance downstream.

4E ROSION PROCESSES WITHIN EPHEMERAL GULLIES To examine the erosion processes within ephemeral gullies, the authors recently conducted a series of Oxpe riments using a 29 m long, 1.8 m wide, and 2.4 m deep horizontal test flume located at the USDAARS location in Stillwater, Oklahoma, U.S.A. (Fig. 2). A red clay soil (CL) with a moisture content of International Journal of Sediment Research, Vol. 15, No. 1, 2000, pp. 31-41 -37-

9.1% was incrementally packed into the flume to a dry density of 1.7 Mg/M3 and a bed slope of 1%. Using a purpose-built tiller, two channels were cut into the packed soil bed. Each trapezoidal channel was approximatel y 0.6 m wide at the top, 0.1 m wide at the bottom, and 0.125 m deep with W1 4.8 and 2:1 sideslopes (27). These dimensions were chosen to replicate closely the physical characteristics of ephemeral gullies described in Table 3. Clear-water flows were measured with an orifice meter and an air-water differential manometer. Test flow durations were typically 2 h. The time variation of channel erosion was closely monitored; morphologic and sediment load data were collected at five cross sections, and bed profiles were measured along the channel centerline at the start and end of each experiment.

0.00 6/2/98

Section 1; 9.1 m

0.24 0.16 0.08

E

Section 2; 10.7m

0.00 0.24

cs

0.16 0.08

.2

^ 31 3.7 rn

0.00

0.16

0.24

Section 4;16.8m

0.00 0.24

0.00 - -After 2.07 It

Section 5; 18.3 rn

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Distance from a datum (m) Fig. 4 Time variation in ephemeral gully morphology for the 6/2/98 experiment. The position of each cross-section is given as distance downstream.

Results from two experiments are shown in Figures 3 through 6. Using a flow rate of 70 gpm (0.00441 m3!s; 6/1/98), channel erosion occurred along the entire channel (Figs. 3 and 5a). Sediment load increased with distance along the channel, and total load decreased with time (Fig. 6). For this experiment, the following morphologic adjustments were observed: (1) the channel bed deepened by 16%, (2) the bottom width enlarged by 350%, (3) the channel sidewalls steepened to 70° in the upstream reaches and to 530 in the downstream reaches, and (4) W/D ratio decreased by 14% to 4.1 (Figs. 3 and 5a). No change was observed in channel top width. Using a higher flow rate (150 gpm or 0.00943 m 3 /s), similar but not identical results were observed for the 6/2/98 experiment. Sediment load increased with distance along the channel and total load decreased with time, but total load was greater for the higher flow rate (Fig. 6). Moreover after 2 h, sediment load did not change appreciably with distance. In the -38-

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upstream reaches, channel depth increased by 26%, bottom width increased by S00%, top width increased by 12%, steepened to 67 (Figs. 4 and Sb). In the downstream reaches, W/D decreased by 13% to 4.2, and the sidewalls420% , and the sideatIs steepened to 49. Clear-water discharge

depth decreased by 17%, bottom width increased b y into these pre-formed swales caused systematic gully erosion: bottom width increased by increased by 16 to 26%, and the gully sidewalls steepened to a maximum o170'. ° (a)



350 to 500%, gully depth

6/1/98 Q=O.00441 in

0.6

05

0-4 >

Oip;

6/2/98 Q=0.00943 4113/S

0.6

0.5 Original bed profile -- Final bed profile

0.4

10 12 14 16

Distance dow nstream (m Fig. 5 Longitudinal profiles of the ephemeral gully bed before and after the (a) 6/1/98 and (b) 6/2/98 experiments. Also given is the flow rate (Q) used.

The transport capacity of the flow played an important role in gully erosion and morphologic adjustment. During experiment 6/1/98, the same ma g nitude of bed adjustment occurred along the entire channel length (Figs. 3 and 5a), and sediment load increased with distance downstream (Fig. 6). Although sediment load decreased with time, one can conclude that the flow was detachment limited, i.e. the flow's transport capacity was never attained during the experiment. In experiment 6/2/98, the higher flow rate caused greater rates of erosion, increasing both the channel width and depth (Figs. 4 and 5b). Excessive sediment transport rates, however, caused the formation of a migrating aggradational (depositional) wedge starting at a distance of 6 m downstream (Fig. 5b), and after 2 h, sediment load reached a constant or asymptotic value at a distance of 7 m from upstream (Fig. 6). Clearly, the flow reached transport capacity sometime between 1 and 2 h, exacerbated by the upstream erosion due to flow entrance effects. Thus the higher sediment yields modulated further channel adjustment in the downstream reaches. Such Interactions between soil erosion and transport capacity would be greatly complicated in gullies on agricultural plots where source areas, flow discharges, and sediment yields are temporally and spatially Varied (see also Thorne et al., 1986). C ONCLUDING REMARKS Ephemeral gully erosion is a significant problem in many geographic regions. The contribution of e phemeral gully erosion to total soil loss typically ranges from 10 to 30% in agricultural regions. Yet erosion prediction technology cannot determine where ephemeral gullies will form and how much soil Will be eroded.

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ER



-39-

0.04

CM

Ch (Y03

— 0,02 s-I

5)

Cr 0.0!

0.0)

0 2 4 6 8 I!) 32 14 16

Distance downstream (m) Fig. 6 At-a-point measurements of sediment load for the ephemeral gully erosion experiments 6/1/98 and 6/2/98, and their temporal variation. Measurements were obtained at each gully cross-section, and are plotted relative to distance downstream. Despite the large differences in climate, watershed size, hydrolo gy, and geography, the ephemeral observed in Spain were morphologically similar to those in Mississippi. On average, Navarrese tly large r, deeper, and wider at the bottom. gullies were sli ghtly longer, and their cross-sections sligh Gullies from Mississippi were wider at the top. These differences may be related to smaller curvature (concavity) of the hollow, smaller watershed slopes, presence of a fragipan, and higher peak flows after C

higher intensity rainfall events observed in Mississippi. Higher

W/D values in Mississippian gullies

(7.7 versus 4.0) appear to be related to intense rainfalls of short duration (Poesen and Govers, 1990). The average volume eroded per gully in Spain was greater (3.18 m) than gullies in Mississippi (1.73 m3 ), and this is presumably due to larger watershed areas with steeper slopes. However, Mississippi experienced much higher rates of erosion (1.68 kg/m 2-y) as compared to Southern Navarra, Spain (0.87 kg/m2-y). Although soils at both localities are considered highly erodible, this difference in erosion rate must be related to the precipitation regime. Each ephemeral gully examined in Mississippi exceeded the tolerable soil erosion limit of 1.2 kg/m-y (USDA-SCS, 1973), and most but not all gullies in Spain also exceeded this limit. The striking similarity of ephemeral gullies in two different geographic locations suggest that gully erosion processes are common, and that technology developed to mitigate such erosion processes may be widely applied. Laboratory experiments revealed that ephemeral gully erosion occurs by bed incision, channel widening, and bank steepening. The flow's transport capacity controlled the magnitude of soil erosion or deposition as well as the time-variation in gully morphology. REFERENCES Auzet, A. V., J. Boiffin, F. Papy, B. Ludwig and J. Maucorps. 1993. Rill erosion as a function of the characteristics of cultivated catchments in the North of France. Catena 20: 41-62. Casali, J. 1997. Caracteri:ación y control de la erosion en cOrcavas. Ph.D. Tessis, Department of Projects and Rural Engineering, Public University ot'Navarra (Spain). Pamplona. Casali, J., J. Lopez and J.V. Giráldez. 1998. Ephemeral gully erosion in Southern Navarra (Spain): description and measurement. Catena in review. Dabney. S.M., C.E. Murphree, L.D. Meyer. E.H. Grissinger, W.C. Harmon and G.B. Triplett. 1997. Runoff and sediment yield from conventional and conservation cropping systems. In i'Ianagement of Landscapes capes Disturbed by Channel Incision. Stabili:ation, Rehabilitation, Restoration, eds. S.S.Y. Wang, E.J. Langendoen and F.D. Shields, 983-989. University of Mississippi. Oxford. -40-

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